We propose to study prospectively the associations between dietary patterns and diet quality and risk of colorectal and breast cancers in cohort studies of 121,700 women aged 30 to 55 years at baseline in 1976 (the Nurses' Health Study; NHS) and 51,529 men aged 40-75 years at baseline in 1986 (the Health Professionals Follow-up Study; HPFS). Detailed dietary data have been collected through validated semiquantitative food frequency questionnaires periodically (every 2-4 years) in both cohorts. Dietary patterns are derived from the food consumption data using factor analysis and cluster analysis. Diet quality is assessed by computing three major dietary indices based on prevailing dietary recommendations, including Diet Quality Index (DQI), Healthy Eating Index (HEI), Recommended Food Score (RFS)). Using the repeated measurements, we will also examine the impact of changes in eating patterns and diet quality over time on subsequent risk of breast and colorectal cancer. In addition, using control samples from previous case-control studies of breast and colorectal cancers, we will evaluate the relationship between dietary patterns and plasma levels of insulin growth factor-1, sex steroid hormones, and other biomarkers of the cancers. Finally, we propose to extend our previous work in diet and breast cancer survival by examining pre- and post-diagnosis dietary patterns and diet quality and breast cancer survival. Our ability to study breast cancer survival as an endpoint has been enhanced by the collection of information on breast cancer treatment and recurrence on all survivors in the year 2000. Overall, the large size of these cohorts, the prospective design, the repeated and detailed measurements of diet and covariates, the high follow-up rates, and the availability of biochemical measurements provide a unique opportunity to study the relationship between overall dietary patterns and the two major cancers in an extremely cost-efficient manner. We believe that this project presents a novel approach to study diet-cancer relationship and will have important public health implications.